A communication volume increases and radio wave interference becomes more aggravated, with the increase of communication devices that utilize the frequency band called unlicensed band (2.4 GHz band). The congestion degree (band occupancy) of the radio wave environment is estimated for the purpose of taking measures against the radio wave interference. If the band occupancy is estimated for each wireless communication standard that uses the unlicensed band, a wireless communication standard having a large influence on the radio wave interference is identified. However, if the signals corresponding to a plurality of wireless communication standards are superimposed in time and frequency domains, and the superimposed signals fail to be separated correctly, there is a risk of erroneously determining the band occupancy of each wireless communication standard.
By the way, there is proposed a method for selecting an event, such as a start and a stop, of transmission by a communication device. In this method, temporal change of reception power in each frequency band is utilized as a reception feature value. In addition, an event, such as a transmission start and a transmission stop, of a communication device and a temporal change pattern of a transmission signal corresponding to the event are utilized as a transmission feature value. An event corresponding to the transmission feature value that is identical with the reception feature value is selected on the basis of comparison between the reception feature value and the transmission feature value.
Moreover, there is disclosed a method for generating information relevant to operation that occurs in a wireless frequency band. This method receives the energy of the wireless frequency band in which operation linked to a plurality of signal types can occur, and generates electric power values of a plurality of frequency components from a digital signal indicating the radio frequency energy received by the wireless frequency band. Also, an average electric power value is compared with a threshold value to generate information of a period during which the signal exists.
See, for example, Japanese Laid-open Patent Publication No. 2008-167200 and U.S. Patent Application Publication No. 2004/0028123.
Time points of a rising portion and a falling portion of signal power are detected by analyzing temporal change of signal power, and time points of a transmission start and a transmission stop are identified from the detected time points. Also, a period during which the signal power is larger than a threshold value is detected by comparing the signal power with the threshold value, in order to identify a period during which the signal exists. However, when a plurality of signals are superimposed and transmitted in the same band and during the same period, it is difficult to separate the individual signals in that band and period, even with the above method.